1. Field of the Invention
This invention relates to a optical disk, especially to an optical disk apparatus for correcting the spherical aberration generated by variation in substrate thickness of an optical disk, and an optical pickup therefor.
2. Description of the Related Art
The optical disk apparatus functions as an information recording/reproducing apparatus that features capability of high-speed access of a large volume of information in a non-contact manner, in a changeable manner, and at a low cost. Making good use of these features, it finds applications as a recording/reproducing apparatus of digital audio signals or digital image signals, or as an external storage device for a computer.
An optical pickup used for this optical disk apparatus generally adopts a configuration comprising: a laser light source of a wavelength corresponding to the optical disk; a diffraction grating for dividing a beam emitted from this light source into three beams; an objective lens for focusing these three beams on an information recording surface of the optical disk with a predetermined NA (Numeral Aperture); a photodetector for converting reflected beams from the optical disk into electric signals; and a beam separating element for guiding these reflected beams to this photodetector, wherein a reproduced information signal and a focusing error signal are detected based on a main beam that is a zero-order diffracted beam of these three reflected beams. And a tracking error signal is detected based on this main beam and sub-beams that are two +first-order diffracted beams. In particular, in the case of a recording optical pickup, it is common to arrange the light spots by the sub beams on the optical disk with a predetermined positional relationship to its track pitch and detect the tracking error signal by the differential push-pull method.
Meanwhile, the optical disk has become denser along with the increase of data amount to be handled with. In contrast to the CD whose capacity is about 700 MB, the DVD with about 4.7 GB has come to be commercialized and widely used. Moreover, a large capacity disk (Blue-ray Disc.) whose capacity is more than 20 GB, for recording high-definition image signals for two hours, are being put to practical use.
The recording density of an optical disk is restricted by the size of a light spot focused on its information recording surface. The size of a light spot is proportional to the wavelength of its beam and inversely proportional to NA (Numerical Aperture); therefore, in order to realize high-density of an optical disk, it is necessary to shorten the wavelength of the beam and enlarge the NA.
Here, it should be noted that when the wavelength of a beam is shortened and the NA is enlarged, aberration increases rapidly due to inclination of the optical disk (hereinafter referred to as “disk inclination”) and an error of the substrate thickness of the optical disk (hereinafter referred to as disk substrate thickness). In addition, in the case of the disk inclination, the comatic aberration increases in proportion to the NA to the third power, to the reciprocal of the wavelength of the beam, and to the disk substrate thickness. For this reason, there is known a technology in which the disk substrate thickness is thinned, responding to increased NA and shortened wavelength. For example, in the Blu-ray Disc, the disk substrate thickness is made thin down to 0.1 mm in comparison to 0.6 mm of the DVD, whereby an allowable margin for the disk inclination is made comparable to that of the DVD.
On the other hand, regarding the error of the disk substrate thickness, the spherical aberration increases in proportion to this error of the disk substrate thickness, the NA to the fourth power, and the reciprocal of the wavelength of the beam. If taking the Blu-ray Disc as an example, an allowable value of the error of the disk substrate thickness will decrease to about 1/10  of that of the DVD. Moreover, in the case of the optical disk with a dual layer structure consisting of two information recording layers, the separation of these information recording layers generates spherical aberration in the same manner as the error of the disk substrate thickness. For this reason, in the high-capacity, high-density optical disk that requires high NA, the correction of the spherical aberration becomes necessary according to its recording layers as well.
For an optical pickup for correcting such spherical aberration, well known is a technology whereby the spherical aberration generated by the error of the disk substrate thickness or the spacing of the information recording layers is canceled by another spherical aberration that can be obtained by adopting anyone of: the first configurations that includes abeam expander for imparting predetermined spherical aberration in an optical path of the beam; the second configuration that includes a liquid crystal device for imparting predetermined spherical aberration; and the third configuration that imparts spherical aberration by changing a lens-to-lens distance of a two-lens objective in a focusing direction.
JP-A No. 11388/2000 proposes that one concrete example of a method for detecting a control signal used for the correction of the spherical aberration and its correction procedure.
The JP-A No. 11388/2000 discloses a technology in which the focus offset is wobbled at a predetermined frequency, or a lens-to-lens distance of a two-lens objective is wobbled at a predetermined frequency lower than this frequency, and the amount of focus offset and the lens-to-lens distance of the objective lens, namely the spherical aberration, are adjusted based on a difference of the amplitude of the information signal RF reproduced from the optical disk between these peaks of wobbling (maximum point and minimum point).
JP-A No. 57616/2000 discloses another method in which the reflected beam from an optical disk is divided and received in inner and outer light-receiving areas of a photodetector, respectively, a signal that varies depending on the spherical aberration is detected as a differential signal of the amounts of light received in the inner and outer light-receiving areas, and the spherical aberration is corrected based on this signal.
The JP-A No. 57616/2000 discloses a technology that provides an optical disk apparatus equipped with an eight-part photodetector—such that each part of the common four-part photodetector is further divided into inner and outer parts—in the detection optics based on the so-called astigmatism method and for obtaining a control signal for correcting the spherical aberration by dividing the reflected beam from the optical disk into inner and outer portions and receiving them.
JP-A No. 307349/2001 discloses another method of using an optical disk apparatus that is equipped with a diffraction grating for separating the reflected beam from an optical disk into two beams, and is configured to receive the respective beams thus separated in inner and outer light-receiving areas of a photodetector independently, and obtain a control signal for correcting the spherical aberration from outputs of this photodetector.
Since JP-A No. 11388/2000 discloses a technology of detecting the spherical aberration by using a wobbling-caused change in the amplitude of a signal reproduced from the optical disk, it cannot be applied to any optical disks in which no information signal RF is recorded, such as a write-once optical disk (write-once disk) in a not-yet-recorded state. Moreover, since it is a technology of detecting the spherical aberration by performing wobbling, if defocus or the spherical aberration is intended to be adjusted while the amount of spherical aberration is being detected, the adjustment will be performed during the wobbling, and hence an optimal adjustment will not be performed. For this reason, it is impossible to detect the amount of spherical aberration during recording/reproduction operations; therefore, it is difficult to detect the spherical aberration in real time during recording/reproduction operations and at the same time perform the correction of the spherical aberration based on this.
With the technology described in the JP-A No. 57616/2000, it is difficult to extract signal lines from light-receiving areas, especially from four inner light-receiving areas of the eight-part photodetector. In addition, since the number of independent signal lines is eight only in the main light-receiving area, the number of necessary amplifiers increases, which brings a problem in terms of size and cost when a photodetector with internal amplifiers (OEIC) is constructed. Furthermore, depending on change in the ambient temperature, a relative positional shift may occur between the reflected beam and a light-receiving surface of the photodetector. In this case, a detected signal of the spherical aberration will have an offset, and hence accurate correction of the spherical aberration becomes difficult. That is, it becomes a problem in terms of reliability against environmental changes.
With the technology disclosed in the JP-A No. 307349/2001, the optical pickup is configured in such a way that four or more light spots are received by a single photodetector, and consequently a light receiving pattern of the photodetector becomes complex and its area becomes large. Even further, since the number of independent signals is large, the number of necessary amplifiers becomes large, and hence it size and cost become a problem when constructing a photodetector with internal amplifiers (OEIC). In addition, there are problems that adjustment of making a plurality of light spots to be received accurately at corresponding positions in the light-receiving surface, respectively, becomes necessary, the adjustment is difficult to achieve, and the adjustment takes a long time.